ConfigurableParam: reject malformed container fields, drop redundant parseSet

[sawenzel]

Stacked on top of AliceO2Group#15525 (cfg/dynamic) — this PR targets that branch, so the diff shown here is only the follow-up changes, not the container support itself.

Three follow-up improvements found while reviewing AliceO2Group#15525, all confined to ContainerParser and its tests:

1. Reject malformed container fields instead of silently dropping them

split() discarded empty fields, so a stray delimiter was swallowed:

• "[1,,3]" parsed to a 2-element vector with no error (element count silently corrupted).
• "key:" dropped the empty map value.

split() now keeps empty tokens, so they are rejected by parseScalar / the map-syntax check and surface as a parse error. The empty-container case ("[]" / "{}") is still short-circuited by the callers before split() runs, so well-formed input is unaffected.

2. Remove the redundant parseSet branch (single-pass parsing)

Every non-map Container already satisfies SequenceContainer, so the parseSet dispatch branch was reached for all sequence and set types and re-parsed the string into a throwaway std::vector before copying element-by-element; the separate else if constexpr (Container<T>) branch was unreachable. parseSequence inserts at end(), which std::set / std::unordered_set accept just as well, so all non-map containers now share one single-pass path. The now-unused HasPushBack concept is removed.

3. Re-enable test cleanup

ConfigurableParam_Container_FileIO_ROOT left test_config.root on disk (the std::remove was commented out); re-enabled to restore test isolation.

Tests

Adds regression coverage:

• set / unordered_set / list / deque parsing through the unified parseSequence path (incl. set de-duplication);
• rejection of empty tokens in both sequence and map parsing.

Note on error severity (intentionally unchanged)

Container parse failures remain non-fatal (logged), which matches the existing scalar setValue behaviour (it already swallows scalar parse errors to std::cerr). This PR does not change that contract — it only stops malformed input from being silently accepted as a different value.

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Verification: the changed parser logic was extracted into a standalone C++20 program and confirmed to compile (g++ -std=c++20 -Wall -Wextra) and behave correctly; full build + boost-test runs are left to CI.

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